A Los Angeles transfusion service physician reports that in his opinion there is no restriction to giving a patient multiple transfusions simultaneously, so long as
one observes some basic rules. First, one must
never mix ABO-incompatible products in the same bag or line. If one were to do so, and if the mixture contained red cells, agglutination might occur due to ABO antibodies. For example, group O platelets should never be mixed in the same line at the same time as group A red cells. Second, if multiple products are administered simultaneously, they
should be given through different lines. As a practical concern, if multiple products are given simultaneously, it might be difficult to know which unit is responsible for a reaction, should a reaction occur.
http://www.cbbsweb.org/enf/2002/multprodsimult.html